Fustrated Total Internal Refexion (FTIR) Bilder

[Ramon]

Ramon (or anyone else), please correct me if I am wrong. Couldn't read the literature yet. Did I get it?

Hi Michael,

I should first say that the name FrTIR, which I have been using, is misleading, because it invites confusion with TIRF. "Frustrated" only describes how the light leaves the guide: the oil between the guide and the sample cancels the total reflection, so the light can pass through. It says nothing about how the specimen is lit, which, as Carsten said, is closer to darkfield with very oblique light.

As for your summary, yes, you got it right for the classic method. The foot on the glass is a very good picture of it. But the gadget is not that method, and it is not TIRF.
In the gadget, the refractive indices of the acrylic and the oil are practically the same, so the light passes from one to the other almost without changing direction or power. The index-matching medium (mostly oil, but also glycerol, paraffin, etc.) creates a continuous path for the light towards the sample.

With opaque samples, the light that goes through the oil strikes the surface of the sample, and what the surface reflects or scatters upwards reaches the objective.

With transparent samples, the light that leaves the guide through the oil reaches the slide, whose refractive index is again almost the same as the oil's, so it carries on towards the sample. What reaches the objective is mainly the light that the sample scatters.

So it is not limited to the points of contact, as in your foot picture. The exception is when the sample is dry or in water, where part or all of the light can't leave the glass, and there your picture comes closer. That was probably the case with my dry diatoms, although this is still a hypothesis.

Finally, what I have always wanted to stress is that this device works both diascopically and episcopically, so with transparent and opaque samples alike. I have always found it interesting that I can examine opaque samples with my old and trusty biological microscope. At least in theory, it can also deliver more grazing light than a standard dry darkfield condenser: illumination NA up to about 1.49, against 0.8–0.95, although I haven't measured this yet. Very grazing light could be interesting for certain samples, for example to bring out fine surface relief and textures. An oil darkfield condenser (NA 1.2–1.4) comes much closer, and I still have to make that comparison.

Ramon

[Sourdough]

Hi Ramon,

thanx a lot! I will keep following all about the method in the Foeum.

Greetings
Michael